1. Field of Invention
This invention pertains to a CMOS driver for providing a relatively constant current independent of large variations of the supply voltage.
2. Description of the Prior Art
Normally a single stage driver or buffer is used to activate one or more digital logic elements from a single logic signal. Typically the maximum current being delivered by the driver varies as the square of the supply voltage. Therefore for logic circuits operating from power supplies which have a fluctuating or poorly regulated output, the buffers must be designed for the worst-case situation, i.e. the lowest expected power supply voltage. For example if a power supply output can vary from 2 V to 7 V and at 2.2 V a typical driver is designed to output 10 ma, the current output of the same driver at 7.2 V will increase to 800 mA. In order to prevent such a drastic increase a current limiter must be used in series with the driver. However such limiters dissipate power continuously increasing the total power of the driver and the integrated chip comprising the driver.